It seems the amount of damage dealt doesn't factor in the amount of experience gained, nor does killing itself does. Every step in battle gives about 0.1 experience

Some easy ways of training experience are:

- Attacking small groups of weak bandits bare handed, this way you don't have to waste time searching bandits, don't waste bullets, hardly get damaged (be sure to move just enough squares so the bandits can't attack you but they do stand right near you after they have moved)